coverage: add tests on remake rules in more complex situations * tests/defs (using_gmake): New function. (for tool in $required): Use it when $tool is 'GNUmake'. * tests/remake-moved-m4-file.test: New test. * tests/remake-deleted-m4-file.test: Likewise. * tests/remake-renamed-m4-file.test: Likewise. * tests/remake-renamed-m4-macro-and-file.test: Likewise. * tests/remake-renamed-m4-macro.test: Likewise. * tests/remake-add-acsubst-gnulib.test: Likewise. * tests/remake-add-header-gnulib.test: Likewise. * tests/remake-remove-header-gnulib.test: Likewise. * tests/Makefile.am (TESTS): Update.